1. Field of the Invention
The present invention relates to an X-ray computed tomographic apparatus which scans a subject by helical scanning, and reconstructs image data in an ECG-gated reconstruction method on the basis of the obtained data.
2. Description of the Related Art
An X-ray computed tomographic apparatus provides information concerning a subject in the form of images on the basis of the intensities of X-rays transmitted through the subject, and plays an important role in many medical activities including diagnoses and medical treatments for diseases, surgical planning, and the like.
It is an important challenge to improve the time resolution of images in examinations on subjects with fast movement, especially in cardiac examinations, which use X-ray computed tomographic apparatuses. A main method for this challenge is a combination of a half reconstruction method and an ECG-gated reconstruction method. As is known, image data is reconstructed by a projection data set acquired while an X-ray tube rotates in the range of 180°+α (where α is the fan angle) centered on the phase of the movement of the heart which is designated by an operator. Recently, attempts have been made to apply a half ECG-gated reconstruction method to helical scanning operation using a detector corresponding to multislice operation (multi-row detector).
FIG. 10 shows a data existing range (the paths of the respective detector rows) together with an electrocardiogram (ECG), with the abscissa representing time and the ordinate representing the position on the Z-axis (body axis). Data sets in the 180°+α ranges (thick lines) centered on designated phases on slices Z1 and Z2 are generated by helical interpolation, and tomographic images are reconstructed from the generated data sets. Likewise, tomographic images of a plurality of slices are reconstructed at predetermined intervals. In an actual examination, by using tomographic images of a plurality of slices, an image of a section in an arbitrary direction crossing them is generated by MPR (Multi-Planar Reformatting).
For example, an image of the slice Z1 originates from a data set at a second heartbeat 2, and an image of the adjacent slice Z2 originates from a data set at a third heartbeat 3. Since an MPR image is generated by partially combining images in different heartbeat periods, banding artifacts (a plurality of discontinuities in an image) are produced in an MPR image due to body movement such as respirations during heartbeat periods, as exemplified by FIG. 7A.